Five general techniques exist in the prior art to control the contrast of photographic transparencies. The five techniques are:
1. Hand dodging; PA1 2. Unsharp markings; PA1 3. Partially reflecting mirrors responsive to an incoherent light source; PA1 4. Infrared quenching of a fluorescent screen excited by an ultraviolet source; and PA1 5. A cathode ray beam with electronic feedback.
In technique (1), a small mask is moved by hand about certain areas of the transparency during a printing operation. The hand dodging method is inexact because of difficulties in manually controlling the position of the small mask on corresponding areas of the transparency. When technique (2) is utilized to make positive prints from negative transparencies, an unsharp positive mask is prepared and, during a printing operation, placed between a printing light source and the negative. While the unsharp masking method enables small areas to be controlled with greater facility than can be attained with the hand dodging method, the unsharp masking method requires an additional development step to produce the mask. Thereby, the unsharp masking method is somewhat inconvenient, since it requires a significant amount of time to implement and it is difficult to control the results.
In technique (3), the original transparency is sandwiched between a pair of partially reflecting mirrors and illuminated through one of the mirrors with a uniform, noncoherent printing light. Since the light is reflected between the mirror surfaces several times, the beam propagated through the mirror located downstream of the transparency has a greater contrast than the transparency. If, however, it is desired to reduce the contrast of the transparency, technique (3) cannot be utilized, without substituting mirrors having different reflecting and transmitting properties. Of course, physical substitution of one mirror for another is inconvenient and time consuming.
In technique (4), a transparency is placed against a fluorescent screen that is illuminated by an ultraviolet source and coated with an infrared quenching phosphor. The negative is illuminated by an infrared source simultaneously with illumination of the screen by the ultraviolet source. In response to the infrared illumination, the screen emits a blue light having a brightness controlled by the quenching action of the infrared source, as coupled through the transparency to the fluorescent screen. Since the transparency is in contact with the fluorescent screen, the transparency modulates the infrared source which in turn modulates the screen brightness. Technique (4) can be employed if it is desired to decrease the contrast of the transparency since the infrared quenching is greatest where the density of the transparency is at a minimum. For this reason, there appears on the fluorescent screen an unsharp image of a polarity type opposite to the polarity of the transparency (e.g., a negative transparency results in an unsharp positive image). Technique (4) has the further disadvantage of being able to only decrease, and not increase, the contrast of the transparency.
In technique (5), a scanning dot of a cathode ray tube illuminates a transparency on the tube face to serve as a printing light for a photographic film being exposed. Behind the film, there is located a photodetector to derive a signal proportional to the transmittance of the transparency. In response to the photodetector output signal, control circuitry for the cathode ray tube is adjusted to modulate either the brightness or the sweep speed of the scanning dot to increase or decrease the transparency contrast. By modulating the scanning dot accordingly, a mask type illumination on the cathode ray tube face, similar to an unsharp positive or negative, is produced. Hence, the only prior art device capable of providing both increased and decreased contrast of photographic transparencies includes very expensive electronic equipment. In addition, the size of the scanning dot on the face of the cathode ray tube provides an upper limit on the resolution of the image.